The present invention relates generally to mask alignment schemes, and more specifically to a mask alignment scheme for forming devices in laterally dielectrically isolated islands.
Devices in integrated circuits are formed by introducing inpurities into a surface of a substrate. This requires the use of a plurality of masks to form regions of different depths and impurity concentrations. Acceptability of the finally formed devices depends upon alignment of the diffused regions. Thus, there is great emphasis on schemes to align a mask to a substrate to assure proper alignment of the diffused regions. For junction isolated substrates, the first diffusion is from a top surface and consequently, all the diffusions may be aligned relative to the first diffusion. This technique may include positioning an indicia on a substrate and performing the first and subsequent diffusions using an indicia on the mask to align with the indicia on the substrate.
For devices formed in dielectrically isolated islands of the type described in U.S. Pat. No. 3,865,649, the dielectrically isolated islands are formed from a first surface which subsequently becomes a buried surface and the devices are formed therein by diffusion into a second surface opposite the first surface which does not exist in the initial material. Thus, no initial mark may be formed which allows alignment of the diffused device regions to the dielectric isolation. Since it is unacceptable for diffused regions of the device to touch the dielectric isolation, more care and time have to be used to align the mask relative to the dielectric isolation. Since the islands have inwardly sloping sides, the surface area available for forming the diffused regions is a function of the amount of starting material removed. Thus, the size of the islands as perceived from the surface varies considerably as illustrated in FIGS. 1a and 1b. At present, the diffusions are aligned to the outside of the islands. In view of the variation in the size, the alignment in a possibly increased sized island will not assure alignment in the remaining islands on the wafer. The varied island size also results in a varying alignment tolerance. This causes difficulty in judging alignment visually.
For devices formed in islands of the substrate having lateral dielectric isolation and a buried junction isolation according to U.S. Pat. No. 3,979,237, the lateral dielectric isolation is formed in the same surface as the devices. Thus, there is a capability of providing an indicia on a common surface for alignment of diffusion. But as with the completely dielectric isolated islands the size of the island as perceived from the surface varies considerably. Thus alignment relative to the lateral dielectric isolation cannot be assured even using an indicia on the same surface for the formation of the lateral isolation and the devices. Thus there exists a need for an alignment technique to be used with laterally and totally dielectrically isolated wafers which assures alignment of the device formation mask with the originally formed islands.